Near-end crosstalk cancellation

ABSTRACT

The present disclosure relates to near-end crosstalk (NEXT) cancellation. A transmit communication signal is transmitted over a first Digital Subscriber Line (DSL) connection using a first group of frequencies and a receive communication signal is received over a second DSL connection using a second group of frequencies that at least partially overlaps the first group of frequencies. A crosstalk correlation between the first and second communication signals is determined. Based on the crosstalk correlation, a crosstalk cancellation signal is generated. The crosstalk cancellation signal is subtracted from the second communication signal, with the intention of reducing NEXT.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Phase application of, and claims thebenefit of, International (PCT) Application Serial No.PCT/CA2017/050550, filed on May 5, 2017, which claims priority to U.S.Patent Application Ser. No. 62/332,580, entitled “NEAR-END CROSSTALKCANCELLATION”, and filed on May 6, 2016, the entire contents of whichare incorporated herein by reference.

FIELD

The present disclosure relates generally to communications and, inparticular, to crosstalk cancellation.

BACKGROUND

Digital Subscriber Line (DSL) technologies such as Very high bit rateDSL (VDSL & VDSL Version 2—VDSL2) rely on isolation between transmit andreceive signals by means of Frequency Division Multiplexing. Alltransmitting modems use specific frequency bands, and all receivingmodems, which are physically in the same chip, use different frequencybands. This ensures that co-located equipment does not suffer Near-EndCrosstalk (NEXT). These frequency bands (groups of frequency carriersthat transmit in the same direction) are referred to as a Band-Plan,with several transmit bands interlaced with receive frequency bands, forexample. However, this type of Band-Plan might not be suitable for allcommunication network deployments.

SUMMARY

According to an aspect of the present disclosure, a communication deviceincludes a transmitter to transmit a first communication signal over afirst DSL connection using a first group of frequencies, a receiver toreceive a second communication signal over a second DSL connection usinga second group of frequencies that at least partially overlaps the firstgroup of frequencies, and a NEXT canceller. The NEXT canceller iscoupled to the transmitter and to the receiver, to determine a crosstalkcorrelation between the first communication signal and the secondcommunication signal, to generate a crosstalk cancellation signal basedon the crosstalk correlation, and to subtract the crosstalk cancellationsignal from the second communication signal.

The first group of frequencies and the second group of frequencies couldfully overlap.

In an embodiment, the NEXT canceller is configured to determine thecrosstalk correlation through a continuous auto-correlation between thefirst communication signal and the second communication signal.

The following is an example of a continuous auto-correlation:∫_(−∞) ^(t)f_(A(t))*f_(B(t))dt,wherein f_(A(t)) and f_(B(t)) are the first communication signal and thesecond communication signal, respectively.

The NEXT canceller could include an Infinite Impulse Response (IIR)filter to generate the crosstalk cancellation signal by filtering thefirst communication signal, and a filter coefficient generator coupledto the IIR filter to generate filter coefficients for the IIR filterbased on the crosstalk correlation.

The NEXT canceller could be configured to generate the filtercoefficients IIR(n) in accordance withIIR(n)=f _(A(t+nT)) ⊗f _(B(t)),wherein f_(A(t+nT)) is the first communication signal delayed by nT,f_(B(t)) is the second communication signal, T is a sample period off_(A(t)) and f_(B(t)), and ⊗ is a correlation integral.

The NEXT canceller could include time delay elements to apply respectivetime delays to the first communication signal, a memory coupled to thetime delay elements to store time delayed versions of the firstcommunication signal, a coefficient generator to generatefrequency-dependent crosstalk coefficients, and multipliers coupled tothe memory and to the coefficient generator to apply thefrequency-dependent crosstalk coefficients to the time delayed versionsof the first communication signal to generate the crosstalk cancellationsignal.

In an embodiment, the NEXT canceller includes a coefficient generator togenerate respective sets of crosstalk coefficients corresponding todifferent relative phase alignments between the first communicationsignal and the second communication signal, a memory coupled to thecoefficient generator to store the crosstalk coefficients, andmultipliers coupled to the memory to apply a set of the crosstalkcoefficients to the first communication signal based on a current phasealignment between the first communication signal and the secondcommunication signal, to generate the crosstalk cancellation signal.

Time Division Multiplexing (TDM) could be applied to communications overthe first and second DSL connections, and first and second far-endcommunication devices respectively coupled to the first and second DSLconnections could be different distances from the communication device.

A communication device could also include: a first transceivercomprising the transmitter and a second receiver to receive a thirdcommunication signal over the first DSL connection using a third groupof frequencies; and a second transceiver comprising the receiver and asecond transmitter to transmit a fourth communication signal over thesecond DSL connection using a fourth group of frequencies that at leastpartially overlaps the third group of frequencies. The NEXT cancellercould be coupled to the first transceiver and to the second transceiver,and be further configured to determine a second crosstalk correlationbetween the fourth communication signal and the third communicationsignal, to generate a second crosstalk cancellation signal based on thesecond crosstalk correlation, and to subtract the second crosstalkcancellation signal from the third communication signal.

The NEXT canceller could include a first NEXT canceller coupled to thefirst transceiver and a second NEXT canceller coupled to the secondtransceiver.

The crosstalk correlation could include a correlation between a furtherinterfering signal and the second communication signal.

Another aspect of the present disclosure provides a method that involvesdetermining a crosstalk correlation between a first communication signalthat is transmitted over a first DSL connection using a first group offrequencies and a second communication signal that is received over asecond DSL connection using a second group of frequencies that at leastpartially overlaps the first group of frequencies. The method alsoincludes generating a crosstalk cancellation signal based on thecrosstalk correlation, and subtracting the crosstalk cancellation signalfrom the second communication signal.

As noted above, the first group of frequencies and the second group offrequencies could fully overlap.

Determining the crosstalk correlation could involve determining thecrosstalk correlation through a continuous auto-correlation between thefirst communication signal and the second communication signal.

The above example of a continuous auto-correlation∫_(−∞) ^(t)f_(A(t))*f_(B(t))dt,could be used in such a method.

Generating the crosstalk cancellation signal could involve generatingfilter coefficients for IIR filtering based on the crosstalkcorrelation, and IIR filtering the first communication signal togenerate the crosstalk cancellation signal.

In an embodiment, generating the filter coefficients involves generatingthe filter coefficients IIR(n) in accordance withIIR(n)=f _(A(t+nT)) ⊗f _(B(t)),as noted above.

Determining the crosstalk correlation could involve applying respectivetime delays to the first communication signal, storing time delayedversions of the first communication signal in a memory, and generatingfrequency-dependent crosstalk coefficients. Generating the crosstalkcancellation signal could then involve applying the frequency-dependentcrosstalk coefficients to the time delayed versions of the firstcommunication signal to generate the crosstalk cancellation signal.

In an embodiment, determining the crosstalk correlation involvesgenerating respective sets of crosstalk coefficients corresponding todifferent relative phase alignments between the first communicationsignal and the second communication signal, and storing the crosstalkcoefficients to a memory, and generating the crosstalk cancellationsignal involves applying a set of the crosstalk coefficients to thefirst communication signal based on a current phase alignment betweenthe first communication signal and the second communication signal, togenerate the crosstalk cancellation signal.

Such a method could be performed at a near-end communication devicecoupled to the first and second DSL connections. TDM could be applied tocommunications over the first and second DSL connections, and first andsecond far-end communication devices respectively coupled to the firstand second DSL connections could be different distances from thenear-end communication device.

A method could also involve: determining a second crosstalk correlationbetween a third communication signal that is received over the first DSLconnection using a third group of frequencies and a fourth communicationsignal that is transmitted over the second DSL connection using a fourthgroup of frequencies that at least partially overlaps the third group offrequencies; generating a second crosstalk cancellation signal based onthe second crosstalk correlation; and subtracting the second crosstalkcancellation signal from the third communication signal to cancelcrosstalk from the third communication signal.

In some embodiments, the crosstalk correlation includes a correlationbetween a further interfering signal and the second communicationsignal.

A non-transitory processor-readable medium could be used to storeinstructions which, when executed by one or more processors, cause theone or more processors to perform a method disclosed herein.

Other aspects and features of embodiments of the present disclosure willbecome apparent to those ordinarily skilled in the art upon review ofthe following description.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of embodiments of the invention will now be described ingreater detail with reference to the accompanying drawings.

FIG. 1 is a block diagram of a typical point-to-point VDSL installation.

FIG. 2A is a block diagram of a communication network in which networknodes include co-located CO/CPE modems.

FIGS. 2B and 2C are block diagrams of other communication networks inwhich network nodes include co-located CO/CPE modems.

FIG. 3 is a block diagram illustrating a co-located CO/CPE modem.

FIG. 4 is a block diagram illustrating a co-located CO/CPE modem with aNEXT canceller according to an embodiment.

FIG. 5 is a block diagram illustrating an embodiment of a NEXTcanceller.

FIG. 6 is a flow diagram illustrating an example method according to anembodiment.

DETAILED DESCRIPTION

FIG. 1 is a block diagram of a typical point-to-point VDSL installation100. In such a typical point-to-point VDSL installation, all co-locatedCentral Office/Optical Node (CO) equipment 110, including CO modems 112,114, uses the same CO-CPE Band-Plan. All remote Customer PremisesEquipment (CPE), including CPE modems 122, 124, uses an inverse CPE-COBand-Plan (transmit versus receive frequency band allocations). ThisBand-Plan arrangement is intended to avoid NEXT.

An alternative network topology is a ring. In a ring, spatial separationof the CO and CPE modems as shown in FIG. 1 might not be practical. Eachnode in a ring network deployment could be CO-equivalent or compatiblein one direction and CPE-equivalent or compatible in another direction.A Band-Plan arrangement as shown in FIG. 1 would not avoid NEXT in sucha ring network deployment, because each node implements both the CO-CPEBand-Plan and the CPE-CO Band-Plan.

FIG. 2A is a block diagram of a communication network 200 in whichnetwork nodes include co-located CO/CPE modems, for example in an xDSLrepeater. Co-located equipment at network node 210 includes a CO modem212 and a CPE modem 214, at network node 220 includes a CO modem 224 anda CPE modem 222, and at network node 230 includes a CO modem 234 and aCPE modem 232. Although only three network nodes 210, 220, 230 are shownin FIG. 2A, a network may include additional nodes. The CPE modem 222and/or the CO modem 234 could be connected to counterpart CO/CPE modemsin other nodes, for example.

FIG. 2B is a block diagram of another communication network in whichnetwork nodes include co-located CO/CPE modems, and represents anexample of a ring topology. The CO modem 234 communicates with the CPEmodem 222 in this example. To avoid congestion in the drawing,Band-Plans are illustrated in FIG. 2B using the same line types as inFIG. 2A, but without text labels. A larger ring could include more thanthe three network nodes, with co-located CO/CPE modems at each networknode communicating with counterpart CPE/CO modems at other nodes in thering.

A ring is an example of a network implementation in which communicationsin overlapping frequency bands could result in NEXT, at the network node210 as shown in FIGS. 2A and 2B, for example. It should be appreciated,however, that CO and CPE modems could potentially be co-located in othertopologies, and NEXT cancellation as disclosed herein could be appliedin any topologies in which NEXT could arise.

The CO modems 212, 224, 234 and the CPE modems 214, 222, 232 could beimplemented as separate modems or integrated into combined CO/CPEmodems. Example CO/CPE modems are shown in FIGS. 3 and 4 and describedbelow. Network connections between the modems at the network nodes 210,220, 230 are by means of DSL connections over twisted wire pairs.

In FIGS. 2A and 2B, communications in one direction use a CO-CPEBand-Plan, and communications in the other direction use the CPE-COBand-Plan. However, with co-location of the CO and CPE modems as shown,NEXT can arise between transmit and receive signals.

According to each Band-Plan as shown in FIGS. 2A and 2B, when a signalis transmitted in a frequency band, it is received in the same frequencyband. Downstream signals transmitted from a CO modem to CPE modem infrequency group A are received by the CPE modem in that same frequencyband. A CPE modem transmits towards a CO modem in a different frequencygroup B in the examples shown, so that upstream signals do not interferewith transmitted downstream signals. In such Frequency DivisionMultiplexing (FDM), one direction of transmission is in one frequencyband, and the other direction of transmission is in a differentfrequency band. Frequency bands are groups of carrier frequencies thatcarry signals in one direction. Each individual carrier frequency ismodulated so that the receiver can ideally reconstruct the original,transmitted signal. Bidirectional communications involves transmissionin both directions along a link.

FDM is one possible approach to using a single physical communicationlink for bidirectional communications. FDM uses different bands offrequencies as described above, and possibly a guard band between thosebands so that the two directions of transmission do not interfere witheach other. Time Division Multiplexing (TDM) is another approach, inwhich a transmitter transmits a signal including a burst of data, and areceiver receives that signal and then sends a burst back in the otherdirection. FDM and TDM could be combined, such that TDM occurs over alarge group of frequencies, as in G.fast for example. It is alsotheoretically possible to apply TDM to FDM signals. Embodimentsdisclosed herein could have application in FDM, TDM, or combined FDM/TDMsystems.

Although it might appear as though a TDM approach as outlined abovemight inherently avoid NEXT between two network nodes, a communicationnetwork could, and typically does, include more than two network nodes.Consider the example shown in FIG. 2C, and an implementation in which anetwork node 250 with co-located modems 252, 254 is at a CO and othernetwork nodes, each with at least one modem 262, 264 are at differentsubscriber premises. The different subscriber premises could becustomer's houses for example, and are different distances X and Y fromthe CO node 210. In an effort to avoid congestion in the drawing, onlyone modem 262, 264 at each subscriber premises is shown in FIG. 2C, buteach subscriber premises could include co-located CO and CPE modems asshown in FIGS. 2A and 2B. One of the modems 252, 254 is a CO modem andthe other is a CPE modem, and similarly the modems 262, 264 arecorresponding CO or CPE modems. For example, the modem 252 could be a COmodem, in which case the modems 254, 262 are CPE modems and the modem264 is a CO modem. The “CO/CPE Modem” and “CPE/CO Modem” labels in FIG.2C are simply intended to encompass other embodiments.

Applying TDM between the CO node 250 and each subscriber premises modem262, 264 could ideally avoid NEXT at the CO node. However, due to thedifferent distances X and Y between the CO node 210 and each subscriberpremises modem 262, 264, communications between the CO node 210 and anyone of the subscriber premises modems 262, 264 could potentiallyinterfere with communications between the CO node and the othersubscriber premises node. Even though each subscriber premises modem262, 264 might not transmit to the CO node 210 until a certain timedelay after receiving a signal from the CO node 210, for example,signals that are being transferred in different directions between theCO node and the subscriber premises modems could still overlap in timeor “pass” each other and thereby interfere with each other. Thispotential interference may arise, for example, due to the differentdistances and thus different transmission times between the CO node 210and the subscriber premises modems 262, 264, and/or different clocktiming at the CO node and one or both of the subscriber premises modems.For example, transmit and receive clocks might not be phase alignedand/or could drift past each other, causing crosstalk from one transmitsymbol that affects one or more receive symbols. NEXT resulting fromsuch effects is also contemplated herein for NEXT cancellation.

FIG. 3 is a block diagram illustrating a current structure of aco-located CO/CPE modem 300. The example modem 300 is a VDSL modem thatincludes a CO-compatible module 310 and a CPE-compatible module 320.Each of the CO-compatible module 310 and the CPE-compatible module 320includes digital circuitry shown by way of example as Digital SignalProcessors (DSPs) 312, 322, and analog circuitry. The analog circuitryincludes an Analog Front End (AFE) 314, 324, transmit (Tx) and receive(Rx) filters 316/318, 326/328 respectively, and hybrid circuits 319,329.

The digital circuitry could be implemented using other types ofcircuitry, in addition to or instead of the DSPs 312, 322. In general,hardware, firmware, components which execute software, or somecombination thereof might be used in implementing the digital circuitry.Electronic devices that might be suitable for implementing any or all ofthese components include, among others, microprocessors,microcontrollers, Programmable Logic Devices (PLDs), Field ProgrammableGate Arrays (FPGAs), Application Specific Integrated Circuits (ASICs),and other types of “intelligent” integrated circuits.

Those skilled in the art will be familiar with various examples ofcomponents that could be used in implementing the analog circuitry.

The DSPs 312, 322 in this example perform digital processing of transmitsignals that are to be transmitted from the modem 300 and receivesignals that are received by the modem. The transmit and receive signalsare exchanged with other network node components. The specific type(s)of such network node components which generate the transmit signals oruse the receive signals will be implementation-specific.

The AFEs 314, 324 convert digital transmit waveforms supplied by theDSPs 312, 322 into analog waveforms for transmission to the line, andalso convert analog receive waveforms from the line into digitalwaveforms for the DSPs. The filters 316/318, 326/328 filter analogsignals, and the hybrid circuits 319, 329 provide interfaces to theline. All of these elements, and various possible implementations, willbe familiar to those skilled in the art.

FIG. 3 also illustrates where NEXT may affect performance of the modem300.

Embodiments of the present disclosure relate to NEXT cancellation. Inone embodiment, a DSP-based cancellation technique is used to cancelinterference arising from NEXT. Although the present disclosure refersto NEXT “cancellation”, it should be appreciated that NEXT might not becancelled entirely. Cancellation is intended to convey the notion ofreducing NEXT, and therefore cancellation encompasses partialcancellation, and not necessarily full elimination of NEXT.

FIG. 4 is a block diagram illustrating a co-located CO/CPE modem 400with a NEXT canceller 430, and shows where the NEXT canceller issituated in one embodiment in order to cancel NEXT.

Many of the components of the example modem 400 could be implemented inthe same way as in the example modem 300. For example, digital circuitryin the example modem 400 could be implemented in DSPs 412, 422 as inFIG. 3, or otherwise as described above with reference to FIG. 3. Theanalog circuitry in the modem 400 could similarly include AFEs 414, 424,Tx and Rx filters 416/418, 426/428 respectively, and hybrid circuits419, 429 as in FIG. 3. All of these components could also operate in thesame way as in FIG. 3. The NEXT canceller 430 is coupled between theDSPs 412, 422 and the AFEs 414, 424 so that it can subtract transmitcrosstalk from receive waveforms, but might not otherwise impactoperation of the other components of the example modem 400.

The NEXT canceller 430, like other digital circuitry in the examplemodem 400, could be implemented using hardware, firmware, componentswhich execute software, or some combination thereof. An example of aNEXT canceller is shown in FIG. 5 and described in detail below. Also,although shown as a separate element in FIG. 4, the NEXT canceller 430could be integrated with one or more other components, such as the DSPs412, 422 and/or other digital circuitry.

In an embodiment, the characteristics of the NEXT are learned by meansof continuous auto-correlation. This is a process whereby the amount ofone signal, that is contained in another signal, can be determined. Thetwo signals are continuously multiplied together and the product isintegrated. If there is no correlation between the two signals, then theintegral will tend toward zero. If there is a correlation, then theintegral will arrive at a value related to the proportion of Signal Athat is contained within Signal B.

The following is an example of a continuous auto-correlation that couldbe used to derive a coefficient:C_(corr)=∫_(−∞) ^(t)f_(A(t))*f_(B(t))dtdenoted asC _(corr) =f _(A(t)) ⊗f _(B(t))where

-   -   f_(A(t)) is the near-end transmit waveform    -   f_(B(t)) is the receive waveform, which may include crosstalk    -   ⊗ is the Correlation Integral    -   C_(corr) is a coefficient derived from the Correlation Integral.

If there is little correlation between the content in different phasesof Signal A, then this technique can be used to determine how much ofSignal A can be found in Signal B at various positions in time. If thistechnique is applied to each of a number of delay values that might beexpected to contain significant crosstalk for example, then a waveformcharacterizing a NEXT transfer function can be derived.

In an embodiment, this procedure applied at multiple delay valuesproduces coefficients for a filter. An Infinite Impulse Response (IIR)filter implementation of a NEXT transfer function, for example, coulduse the following filter coefficients:IIR(n)=f _(A(t+nT)) ⊗f _(B(t))where

-   -   IIR(n) are the IIR coefficients    -   f_(A(t+nT)) is the near-end transmit waveform f_(A(t)), delayed        by nT    -   f_(B(t)) is the receive waveform, which may include crosstalk    -   T is the sample period of the digital waveforms f_(A(t)) and        f_(B(t))    -   ⊗ is the Correlation Integral.

The expected NEXT signal, also referred to herein as a crosstalkcancellation signal, is derived in this embodiment by passing thenear-end transmit waveform through an IIR filter using the derivedcoefficients. The expected NEXT signal is then subtracted from thereceive waveform to ideally recover the original waveform that wastransmitted by a far-end transmitter, or in practical terms to reducethe effect of crosstalk on the far-end transmit waveform and recover areceive waveform that is closer to the original far-end transmitwaveform. Other implementations, using other types of filters, forexample, are also possible.

FIG. 5 is a block diagram illustrating an embodiment of a NEXT canceller500. The example NEXT canceller 500 includes N delay elements 510-1 to510-N, which apply successive delays of one sample period to a transmitsignal. N could be determined, for example, based on signal delay on atransmit and/or receive path, between a modem's transceiver and othercomponents of a network node. In an embodiment, N is based on the signaldelay or distance between the modem and a relay card in the networknode. N could also or instead be a function of one or more otherparameters, such as available resources in an FPGA/ASIC or space in amemory to store samples, an amount of tolerable delay through the NEXTcanceller, an amount of processing power available for the NEXTcanceller, the degree of overlap between transmit and receive frequencybands, etc.

For each delayed version of the transmit signal, representingpotentially different timing or phase alignments between a receivedsignal and a near-end transmitted signal, a respective one of Ncorrelators determines a correlation between the delayed transmit signaland a receive signal. Each correlator includes a multiplier 512-1 to512-N and an integrator 514-1 to 514-N. The correlators are an exampleof a filter coefficient generator. The resulting IIR filter coefficientsare provided to an IIR filter 516, and the filtered transmit signal issubtracted from the receive signal by a subtractor 518.

Various implementations of the components shown in FIG. 5 are possible,and those skilled in the art will be familiar with examples of delayelements, multipliers, integrators, IIR filters, and subtractors thatcould be used to implement the components shown in FIG. 5.

In another embodiment, a NEXT canceller includes time delay elementssuch as 510-1 to 510-N, to apply respective time delays to the transmitsignal. A memory, which could include one or more memory devices, iscoupled to the time delay elements to store time delayed versions of thetransmit signal. A coefficient generator generates frequency-dependentcrosstalk coefficients based on correlation between each delayedtransmit signal and the receive signal, and multipliers are coupled tothe memory and to the coefficient generator to apply thefrequency-dependent crosstalk coefficients to the time delayed versionsof the transmit signal, to thereby generate a crosstalk cancellationsignal. In this embodiment, there could be two sets of multipliers,including the multipliers shown 512-1 to 512-N as part of a coefficientgenerator and separate multipliers to apply generated coefficients tothe time delayed versions of the transmit signal, or one set ofmultipliers could be used at different times for coefficient generationand application of coefficients to the time delayed versions of thetransmit signal. Different time delays could be applicable, for example,in a scenario in which transmit and receive clocks are not perfectlyphase aligned and/or may drift past each other, causing crosstalk fromone transmit symbol that affects one or more receive symbols.

According to another embodiment, a coefficient generator could generaterespective sets of crosstalk coefficients corresponding to differentrelative phase alignments between a near-end transmit signal and areceive signal. These sets of coefficients could be stored in a memory.The memory could implement additional memory banks in addition to thosethat store, for example, delayed versions of the transmit signal.Multipliers coupled to the memory apply a set of the crosstalkcoefficients to the transmit signal based on a current phase alignmentbetween the transmit signal and the receive signal, to generate thecrosstalk cancellation signal. Current phase alignment could bedetermined, for example, by recovering the receive clock from anincoming signal, using a Phase Locked Loop (PLL), Surface Acoustic Wave(SAW) device, or other clock recovery mechanism, and comparing relativephases of the recovered clock's edges with edges of the transmit clock.The coefficients for different time delays corresponding to differentphase shifts could be pre-calculated using a training pattern or othercalibration sequence, for example.

Different phase alignments or different phase relationships betweennear-end transmit signals and receive signals could arise, for example,even in implementations that apply TDM to communications over differentDSL connections. Far-end communication devices coupled to those DSLconnections could be at different distances from the near-end device,leading to different timing between the far-end devices. In thisscenario, described above with reference to FIG. 2B by way of example,the transmit and receive signals overlap in the time domain, implying aphase relationship between the signals that are used in NEXTcancellation.

Various embodiments are described in detail above. More generally, acommunication device such as a CO/CPE modem includes a transmitter totransmit a first communication signal over a first DSL connection usinga first group of frequencies, and a receiver to receive a secondcommunication signal (from a CO modem in FIG. 4, for example) over asecond DSL connection using a second group of frequencies that at leastpartially overlaps the first group of frequencies. The firstcommunication signal could be a signal that is transmitted to a CPEmodem over the “top” DSL connection in FIG. 4, or a signal that istransmitted to a CO modem over the “bottom” DSL connection in FIG. 4,for example. Similarly, the second communication signal could be asignal that is received from a CPE modem over the “top” DSL connectionin FIG. 4, or a signal that is received from a CO modem over the“bottom” DSL connection in FIG. 4. The first and second groups offrequencies could fully overlap as shown in FIG. 2, for example, or onlypartially overlap and include one or more common frequencies.

Such a communication device also includes a NEXT canceller, coupled tothe transmitter and to the receiver. The NEXT canceller is configured todetermine a crosstalk correlation between the first communication signaland the second communication signal. The crosstalk correlation could bedetermined through a continuous auto-correlation between the firstcommunication signal and the second communication signal, and an exampleof such a correlation is provided above.

The NEXT canceller is also configured to generate a crosstalkcancellation signal based on the crosstalk correlation. In an embodimentdescribed above, the NEXT canceller includes an IIR filter 516 (FIG. 5)to generate the crosstalk cancellation signal by filtering the firstcommunication signal, and the NEXT canceller is configured to generatefilter coefficients for the IIR filter based on the crosstalkcorrelation. An example approach for generating the IIR filtercoefficients is provided above.

The NEXT canceller subtracts the crosstalk cancellation signal from thesecond communication signal.

The transmitter could be part of a first transceiver, such as a CO/CPEmodem, that also includes a receiver to receive a third communicationsignal over the first DSL connection using a third group of frequencies.Similarly, the receiver that receives the second communication signalcould be part of a second transceiver that also includes a secondtransmitter to transmit a fourth communication signal over the secondDSL connection using a fourth group of frequencies that at leastpartially overlaps, and may fully overlap, the third group offrequencies. In an embodiment, the third group of frequencies is thesame as the second group of frequencies, and the fourth group offrequencies is the same as the first group of frequencies, so that thereare two groups of frequencies as shown in FIG. 3.

The NEXT canceller could be coupled to the first transceiver and to thesecond transceiver as shown in FIG. 4, for example, and be furtherconfigured to determine a second crosstalk correlation between thefourth communication signal and the third communication signal, togenerate a second crosstalk cancellation signal based on the secondcrosstalk correlation, and to cancel crosstalk from the thirdcommunication signal by subtracting the second crosstalk cancellationsignal from the third communication signal. The NEXT canceller in FIG.5, for example, could be coupled to both DSPs 412, 422 and to both AFEs414, 424 (FIG. 4), to cancel NEXT from signals that are received from aremote CPE modem or a remote CO modem. A NEXT canceller could insteadinclude separate NEXT cancellation circuits such as the circuit shown inFIG. 5, to cancel NEXT from signals that are received on each of the tworeceive paths.

The embodiments described above relate to communication devices such asmodems. Method embodiments are also contemplated.

FIG. 6 is a flow diagram illustrating an example method 600. In themethod 600, communication signals are transmitted and received at 602. Acrosstalk correlation, between a first communication signal that istransmitted over a DSL connection using a first group of frequencies anda second communication signal that is received over a second DSLconnection using a second group of frequencies that at least partiallyoverlaps the first group of frequencies, is determined at 604. Acrosstalk cancellation signal is generated at 606, based on thecrosstalk correlation. The crosstalk cancellation signal is subtractedfrom the second communication signal at 608, and the resultant signal isoutput at 610, for further receiver processing for example.

The example method 600 is illustrative of one embodiment. Examples ofadditional operations that may be performed, and examples of howoperations may be performed, will be apparent from the description anddrawings relating to modems or implementations, for example. A methodneed not be performed only once as shown in FIG. 6, but could berepeated or ongoing as a continuous process. Further variations may beor become apparent.

What has been described is merely illustrative of the application ofprinciples of embodiments of the present disclosure. Other arrangementsand methods can be implemented by those skilled in the art.

For example, the examples in FIGS. 4 to 6 are intended solely forillustrative purposes. The present invention is in no way limited to theparticular example embodiments explicitly shown in the drawings anddescribed herein.

In some embodiments, a NEXT canceller could be operated in an initialtraining period during which it learns the characteristics of the NEXTthat is to be cancelled. Next cancellation could then commence after thetraining period.

A NEXT canceller could potentially identify other interference sourcesor signals as well, and not only a transmit signal that is transmittedby co-located equipment. NEXT and other interference could then becancelled. For example, there could be side contributions from knownrepetitive signals that have somewhat consistent correlations, and thesecould be taken into account as well in determining cancellation signals.For example, a crosstalk correlation could include a correlation betweena further interfering signal, in addition to a transmit signal, and areceive signal from which NEXT is to be cancelled. Any interferingsignal(s), from one or more interference source(s), could be consistentor predictable, or possibly detected, and included in determining acorrelation with the receive signal. The signal f_(A(t)) could includenot only a transmit signal, but a combination of the transmit signal andone or more interfering signals, for example. A correlation contributionfrom the interfering signal(s) could also or instead be otherwisedetermined and included in the crosstalk correlation that is used inNEXT cancellation. In some embodiments, the interfering signal(s) couldbe time-varying, but consistent or predicable at least within thetimeframe of NEXT cancellation coefficients. In this manner,interference arising from additional interfering signals and othersources of interference could also be cancelled.

In addition, although described primarily in the context of methods andsystems, other implementations are also contemplated, as instructionsstored on a non-transitory computer-readable medium, for example.

We claim:
 1. A communication device comprising: a transmitter totransmit a first communication signal over a first Digital SubscriberLine (DSL) connection using a first group of frequencies; a receiver toreceive a second communication signal over a second DSL connection usinga second group of frequencies that at least partially overlaps the firstgroup of frequencies; a Near-End Crosstalk (NEXT) canceller, coupled tothe transmitter and to the receiver, to determine a crosstalkcorrelation between the first communication signal and the secondcommunication signal, to generate a crosstalk cancellation signal basedon the crosstalk correlation, and to subtract the crosstalk cancellationsignal from the second communication signal, wherein the NEXT cancellercomprises an Infinite Impulse Response (IIR) filter to generate thecrosstalk cancellation signal by filtering the first communicationsignal, and a filter coefficient generator coupled to the IIR filter togenerate filter coefficients for the IIR filter based on the crosstalkcorrelation.
 2. The communication device of claim 1, wherein the NEXTcanceller is configured to determine the crosstalk correlation through acontinuous auto-correlation between the first communication signal andthe second communication signal.
 3. The communication device of claim 2,wherein the continuous auto-correlation comprises∫_(−∞) ^(t)f_(A(t))*f_(B(t))dt, wherein f_(A(t)) and f_(B(t)) are thefirst communication signal and the second communication signal,respectively.
 4. The communication device of claim 1, wherein the NEXTcanceller is configured to generate the filter coefficients IIR(n) inaccordance withIIR(n)=f _(A(t+nT)) ⊗f _(B(t)), wherein f_(A(t+nT)) is the firstcommunication signal delayed by nT, f_(b(t)) is the second communicationsignal, T is a sample period of f_(A(t)) and f_(B(t)), and ⊗ is acorrelation integral.
 5. The communication device of claim 1,comprising: a first transceiver comprising the transmitter and a secondreceiver to receive a third communication signal over the first DSLconnection using a third group of frequencies; a second transceivercomprising the receiver and a second transmitter to transmit a fourthcommunication signal over the second DSL connection using a fourth groupof frequencies that at least partially overlaps the third group offrequencies, wherein the NEXT canceller is coupled to the firsttransceiver and to the second transceiver, and is further configured todetermine a second crosstalk correlation between the fourthcommunication signal and the third communication signal, to generate asecond crosstalk cancellation signal based on the second crosstalkcorrelation, and to subtract the second crosstalk cancellation signalfrom the third communication signal.
 6. The communication device ofclaim 5, wherein the NEXT canceller comprises a first NEXT cancellercoupled to the first transceiver and a second NEXT canceller coupled tothe second transceiver.
 7. The communication device of claim 1, whereinthe crosstalk correlation includes a correlation between a furtherinterfering signal and the second communication signal.
 8. A methodcomprising: determining a crosstalk correlation between a firstcommunication signal that is transmitted over a first Digital SubscriberLine (DSL) connection using a first group of frequencies and a secondcommunication signal that is received over a second DSL connection usinga second group of frequencies that at least partially overlaps the firstgroup of frequencies; generating a crosstalk cancellation signal basedon the crosstalk correlation; subtracting the crosstalk cancellationsignal from the second communication signal, wherein generating thecrosstalk cancellation signal comprises: generating filter coefficientsfor Infinite Impulse Response (IIR) filtering based on the crosstalkcorrelation; IIR filtering the first communication signal to generatethe crosstalk cancellation signal.
 9. The method of claim 8, whereindetermining the crosstalk correlation comprises determining thecrosstalk correlation through a continuous auto-correlation between thefirst communication signal and the second communication signal.
 10. Themethod of claim 9, wherein the continuous auto-correlation comprises∫_(−∞) ^(t)f_(A(t))*f_(B(t))dt, wherein f_(A(t)) and f_(B(t)) are thefirst communication signal and the second communication signal,respectively.
 11. The method of claim 8, wherein generating the filtercoefficients comprises generating the filter coefficients IIR(n) inaccordance withIIR(n)=f _(A(t+nT)) ⊗f _(B(t)), wherein f_(A(t+nT)) is the firstcommunication signal delayed by nT, f_(B(t)) is the second communicationsignal, T is a sample period of f_(A(t)) and f_(B(t)), and ⊗ is acorrelation integral.
 12. The method of claim 8, further comprising:determining a second crosstalk correlation between a third communicationsignal that is received over the first DSL connection using a thirdgroup of frequencies and a fourth communication signal that istransmitted over the second DSL connection using a fourth group offrequencies that at least partially overlaps the third group offrequencies; generating a second crosstalk cancellation signal based onthe second crosstalk correlation; subtracting the second crosstalkcancellation signal from the third communication signal to cancelcrosstalk from the third communication signal.
 13. The method of claim8, wherein the crosstalk correlation includes a correlation between afurther interfering signal and the second communication signal.
 14. Anon-transitory processor-readable medium storing instructions which,when executed by one or more processors, cause the one or moreprocessors to perform the method of claim 8.